35 research outputs found
Growth, transport, and magnetic properties of Pr0.67Ca0.33MnO3 thin films
We have grown Pr0.67Ca0.33MnO3 thin films on LaAlO3 using pulsed laser deposition. Below 50 K, a field induced insulator-metal transition results in changes in resistivity of at least 6 orders of magnitude. The field induced conducting state is metastable at low temperature. The temperature dependence of the resistivity exhibits considerable hysteresis in a field of 40 kOe but becomes reversible in a field of 80 kOe
Phase diagram for Ca_{1-x}Y_xMnO_3 type crystals
We present a simple model to study the electron doped manganese perovskites.
The model considers the competition between double exchange mechanism for
itinerant electrons and antiferromagnetic superexchange interaction for
localized electrons. It represents each Mn^{4+} ion by a spin 1/2, on which an
electron can be added to produce Mn^{3+}; we include a hopping energy t, a
strong intratomic interaction exchange J (in the limit J/t>>1), and an
interatomic antiferromagnetic interaction K between the local spins. Using the
Renormalized Perturbation Expansion and a Mean Field Approximation on the
hopping terms and on the superexchange interaction we calculate the free
energy. From it, the stability of the antiferromagnetic, canted, ferromagnetic,
and novel spin glass phases can be determined as functions of the parameters
characterizing the system. The model results can be expressed in terms of t and
K for each value of the doping x in phase diagrams. The magnetization m and
canting angle can also be calculated as fuctions of temperature for fixed
values of doping and model parameters.Comment: 4 figure
Effect of disorder on the magnetic and transport properties of La_{1-x}Sr_{x}MnO_{3}
We study a simplified model of the electronic structure of compounds of the
type of LaSrMnO. The model represents each Mn ion by a
spin S=1/2, on which an electron can be added to produce Mn. We include
two strong intratomic interactions in the Hamiltonian: exchange (% ) and
Coulomb (). Finally, to represent the effect of Sr substitution by La in a
simple way, we include a distribution of diagonal energies at the Mn sites.
Then we use Green function techniques to calculate a mobility edge and the
average density of states. We find that according to the amount of disorder and
to the concentration of electrons in the system, the Fermi level can cross the
mobility edge to produce a metal to insulator transition as the magnetization
decreases (increase of temperature). If the disorder is large, the system
remains insulating for all concentrations. Concentrations near zero or one
favor the insulating state while intermediate values of concentration favor the
metallic state.Comment: 11 pages, 4 figures available upon request, accepted for publication
in Solid State Communication
Electron-Doped Manganese Perovskites: The Polaronic State
Using the Lanczos method in linear chains we study the ground state of the
double exchange model including an antiferromagnetic super-exchange in the low
concentration limit. We find that this ground state is always inhomogeneous,
containig ferromagnetic polarons. The extention of the polaron spin distortion,
the dispersion relation and their trapping by impurities, are studied for
diferent values of the super exchange interaction and magnetic field. We also
find repulsive polaron polaron interaction.Comment: 4 pages, 6 embedded figure
Zone center phonons of the orthorhombic RMnO3 (R = Pr, Eu, Tb, Dy, Ho) perovskites
A short range force constant model (SRFCM) has been applied for the first
time to investigate the phonons in RMnO3 (R = Pr, Eu, Tb, Dy, Ho) perovskites
in their orthorhombic phase. The calculations with 17 stretching and bending
force constants provide good agreement for the observed Raman frequencies. The
infrared frequencies have been assigned for the first time.
PACS Codes: 36.20.Ng, 33.20.Fb, 34.20.CfComment: 8 pages, 1 figur
Intermediate Valence Model for the Colossal Magnetoresistance in Tl_{2}Mn_{2}O_{7}
The colossal magnetoresistance exhibited by Tl_{2}Mn_{2}O_{7} is an
interesting phenomenon, as it is very similar to that found in perovskite
manganese oxides although the compound differs both in its crystalline
structure and electronic properties from the manganites. At the same time,
other pyrochlore compounds, though sharing the same structure with
Tl_{2}Mn_{2}O_{7}, do not exhibit the strong coupling between magnetism and
transport properties found in this material. Mostly due to the absence of
evidence for significant doping into the Mn-O sublattice, and the tendency of
Tl to form conduction bands, the traditional double exchange mechanism
mentioned in connection with manganites does not seem suitable to explain the
experimental results in this case. We propose a model for Tl_{2}Mn_{2}O_{7}
consisting of a lattice of intermediate valence ions fluctuating between two
magnetic configurations, representing Mn-3d orbitals, hybridized with a
conduction band, which we associate with Tl. This model had been proposed
originally for the analysis of intermediate valence Tm compounds. With a
simplified treatment of the model we obtain the electronic structure and
transport properties of Tl_{2}Mn_{2}O_{7}, with good qualitative agreement to
experiments. The presence of a hybridization gap in the density of states seems
important to understand the reported Hall data.Comment: 8 pages + 5 postscript fig
Double Degeneracy and Jahn-Teller Effects in CMR Perovskites
Jahn-Teller (JT) electron-phonon coupling effects in the colossal
magnetoresistance perovskite compounds are investigated.
Electron-electron correlations between two degenerate Mn orbitals are
studied in the Gutzwiller approximation. The static JT distortion and
antiadiabatic polaron effects are studied in a modified Lang-Firsov
approximation. We find that (i) the electron or hole character of the charge
carrier depends on the static JT distortion, and (ii) due to the two-component
nature of the JT coupling, fluctuations in the JT distortion direction
contribute to the charge transport in similar fashion as the local spins.Comment: 11 RevTeX pages. 3 Figures available upon request. submitted to Phys.
rev. B (Rapid Communications
Charge Localization in Disordered Colossal-Magnetoresistance Manganites
The metallic or insulating nature of the paramagnetic phase of the
colossal-magnetoresistance manganites is investigated via a double exchange
Hamiltonian with diagonal disorder. Mobility edge trajectory is determined with
the transfer matrix method. Density of states calculations indicate that random
hopping alone is not sufficient to induce Anderson localization at the Fermi
level with 20-30% doping. We argue that the metal-insulator transtion is likely
due to the formation of localized polarons from nonuniform extended states as
the effective band width is reduced by random hoppings and electron-electron
interactions.Comment: 4 pages, RevTex. 4 Figures include
Ferromagnetic Polarons in Manganites
Using the Lanczos method in linear chains we study the double exchange model
in the low concentration limit, including an antiferromagnetic super-exchange
K. In the strong coupling limit we find that the ground state contains
ferromagnetic polarons whose length is very sensitive to the value of K/t. We
investigate the dispersion relation, the trapping by impurities, and the
interaction between these polarons. As the overlap between polarons increases,
by decreasing K/t, the effective interaction between them changes from
antiferromagnetic to ferromagnetic. The scaling to the thermodynamic limit
suggests an attractive interaction in the strong coupling regime (J_h > t) and
no binding in the weak limit (J_h \simeq t).Comment: 12 pages, accepted in PRB, to be published in Novembe
Optical Conductivity of Manganites: Crossover from Jahn-Teller Small Polaron to Coherent Transport in the Ferromagnetic State
We report on the optical properties of the hole-doped manganites Nd_{0.7}Sr
_{0.3}MnO_{3}, La_{0.7}Ca_{0.3}MnO_{3}, and La_{0.7}Sr_{0.3}MnO_{3}. The
low-energy optical conductivity in the paramagnetic-insulating state of these
materials is characterized by a broad maximum near 1 eV. This feature shifts to
lower energy and grows in optical oscillator strength as the temperature is
lowered into the ferromagnetic state. It remains identifiable well below Tc and
transforms eventually into a Drude-like response. This optical behavior and the
activated transport in the paramagnetic state of these materials are consistent
with a Jahn-Teller small polaron. The optical spectra and oscillator strength
changes compare well with models that include both double exchange and the
dynamic Jahn-Teller effect in the description of the electronic structure.Comment: 27 pages (Latex), 6 figures (PostScript